1. Field of the Invention
The present invention relates to a sealed type alkaline storage battery such as a nickel-metal hydride storage battery, a nickel-cadmium storage battery or the like, and more particularly to optimal adjustment of a discharge capacity ratio and an area ratio of positive and negative electrode plates in the alkaline storage battery.
2. Description of the Prior Art
In a sealed type alkaline storage battery such as a nickel-metal hydride storage battery, a nickel-cadmium storage battery or the like, the discharge capacity of a negative electrode plate is adjusted larger than that of a positive electrode plate to restrain hydrogen gas generated from the negative electrode plate at charging and to absorb oxygen gas generated from the positive electrode plate thereby to restrain an increase of internal pressure of the battery.
In general, a sealed type alkaline storage battery of this kind includes a group of spiral electrodes composed of sheet-like positive and negative electrode plates spirally wound through a separator in such a manner that the negative electrode plate is placed at an outermost periphery of the group of spiral electrodes whereas the positive electrode is placed at an innermost periphery of the group of spiral electrodes In such a structure of the group of spiral electrodes, sufficient utilization of an active material in the positive electrode plate may not be effected since the positive electrode plate is not opposed to the negative electrode plate at its one surface The reaction area of the electrode plates is limited in a small extent at the innermost periphery of the group of spiral electrodes, and the capacity of the positive electrode plate becomes larger than that of the negative electrode plate Due to such imbalance of the positive and negative electrode plates in discharge capacity, the negative electrode plate does not effect sufficient absorption of oxygen gas generated from the positive electrode plate at over-charging, and hydrogen equilibrium potential increases. This causes an increase of internal pressure of the storage battery. Particularly, such a problem is raised in the nickel-metal hydride storage battery since the generation amount of hydrogen gas increases un accordance with equilibrium pressure of hydrogen gas. Accordingly, to increase the life span of the sealed type alkaline storage battery, it is indispensable to restrain the generation of hydrogen gas in balance with absorption of oxygen gas.
A primary object of the present invention is to provide a sealed type alkaline storage battery wherein a discharge capacity ratio and an area ratio of positive and negative electrode plates are optimized to restrain the generation of hydrogen gas in balance with absorption of oxygen gas thereby to increase the life span of the storage battery.
According to the present invention, the object is accomplished by providing a sealed type alkaline storage battery in which entire surface of a positive electrode plate is opposed to a negative electrode plate through a separator, wherein a discharge capacity ratio of the negative electrode plate to the positive electrode plate is determined to be more than or equal to 1.9, an area ratio of the negative electrode plate to the positive electrode plate is determined to be less than or equal to 1.4, and the thickness of the positive electrode plate is determined to be less than or equal to 0.6 mm.
By the fact that the discharge capacity ratio of the negative electrode plate to the positive electrode plate is more than or equal to 1.9, the hydrogen absorbing property at charging is enhanced to restrain the generation of hydrogen gas from the negative electrode plate. By the fact that the area ratio of the negative electrode plate to die positive electrode plate is less than or equal to 1.4, the area of the negative electrode plate opposed to the positive electrode plate is increased to enhance the reducing absorption property of oxygen gas generated from the positive electrode plate at charging. In addition, by the fact that the thickness of the positive electrode plate is less than or equal to 0.6 mm, the oxygen gas generated at charging is effectively transferred to the surface of the negative electrode plate to enhance the reducing absorption property of oxygen gas at the negative electrode plate. As a result an increase of internal pressure of the storage battery caused by accumulation of hydrogen gas and oxygen gas is restrained to enhance the continual discharge characteristic of the storage battery.
In a practical embodiment of the present invention, it is preferable that nickel hydroxide covered with cobalt compound containing sodium is used as an active material of the positive electrode plate to increase the generation potential of oxygen gas thereby to restrain the generation of oxygen gas and to enhance the charging property of nickel hydroxide. As the cobalt compound containing sodium is superior in conductivity, the conductivity between particles of nickel hydroxide is enhanced to increase the life span of the storage battery. In addition, in the case that the nickel hydroxide covered with the cobalt compound containing sodium is added with at least one of oxide powders of yttrium (Y), Ytterbium (Yb), erbium (Er) and gadolinium (Gd), the generation potential of oxygen gas is increased by the multiplication effect of the cobalt compound and the oxide powder to restrain the generation of oxygen gas and to enhance the charging property of nickel hydroxide.